In detecting the near proximity between objects such as a projectile and a target wherein at least one of the objects and possibly both objects are moving at high speed, it is often desirable to activate circuitry within the projectile at a selectable distance from the target before impact occurs. As flight speeds and closing speeds increase these distances for activation are also subject to change. Accordingly the use of such devices as a proximity switch, ogive crush switch, or extensible probe, while functioning satisfactorily on contact with a target are not reliable for operation at distances beyond one or two feet and can have an adverse aerodynamic influence during projectile or vehicle flight. Accordingly, longer standoff distances may often be desirable.
Rangefinders are well established for detecting long range distances between objects. A coventional RF or optical rangefinder consists of a transmitter, target and receiver. The receiver measures the round trip transit time of the transmitted electromagnetic energy reflected back from the target. This provides an accurate, automatic long range rangefinder. However, at distances of less than approximately ten feet, such rangefinders become quite difficult to implement. At these very short ranges, a pulse transmitter must be limited to a nanosecond or less output, and the receiver (co-located with the transmitter) must recover from undesired reflected signal overload, electromagnetic interference and other transients within a similar time.
Another method consists of two optical systems, not coaxial, which produce a single blurred image of a target in which the blurring is caused by parallax. Changing the angle between the two optical systems causes the parallax to disappear for one range, providing a sharp image of a target at that one range. Measuring the new angle provides an indication of range for very short to moderate range targets. This method is most useful for ranges from 10 to 1,000 times the separation between the optical axes. It is, however, a complex task to automatically perform ranging in this method, due to the complex judgments involved in "best focus" of a target image.